1. Field of the Invention
This invention relates to a protective circuit unit for an electronic device, and more particularly to a protective circuit unit for a mobile electronic device installed on a motor vehicle or the like.
2. Related Art
Sometimes, a mobile electronic device operated on the battery of a motor vehicle suffers from a difficulty that an abnormal voltage higher than the rated voltage such as 12 V or 24 V may be applied to it depending on, for instance, the variations in speed of the engine. In order to protect the electronic device from being damaged by the abnormal voltage, a protective circuit is provided for the mobile electronic device.
A protective circuit is as shown in FIG. 3. In FIG. 3, reference numeral 1 designates a positive power line; 2, an input terminal connected to the positive electrode of a battery; and 3, an output terminal connected to the power circuit of the electronic device. A fuse 4, a varistor 5, a surge absorbing filter made up of a choke coil 6 and a capacitor 7, and diode 8 are connected between the input terminal 2 and the output terminal 3 in the stated order. The diode 8 is to prevent the electronic circuit from being damaged when the power source is connected with the polarity reversed.
The electrical system of a motor vehicle sometimes suffers from a surge voltage of the order of 100 V to 200 V depending on, for instance, the variations in speed of the engine. Therefore, the varistor 5 is employed to limit the input voltage to about 80 V to 100 V. In the case where the abnormally high voltage lasts for a relatively long time, the fuse is blown to protect the electronic circuit. An impulsive surge voltage is absorbed by the surge absorbing filter made up of the choke coil 6 and the capacitor 7. In addition, in the electronic device, the withstand voltage of the power circuit is set to around 100 V.
That is, in the conventional mobile electronic device, the withstand voltage of the power circuit is set to about 100 V with the absorption of surge voltage made by the choke coil and the capacitor taken into consideration. As a result, in the mobile electronic device, the components are bulky, and the power source is also bulky. Hence, the mobile electronic device is unavoidably high in manufacturing cost.